1. Field of the Invention
The present invention relates to optical limiters and particularly to a hybrid passive optical limiter for protecting eyes and sensors from intense visible and near infrared laser radiation by utilizing a thermal-defocusing mechanism to limit light within a first intensity range and by utilizing a nonlinear scattering mechanism to limit light having an intensity above the first intensity range.
2. Description of the Related Art
The protection of eyes and sensors from damage due to sources of intense light, such as laser radiation, is a problem of current interest in both commercial and military environments. Nonlinear optical materials (materials whose optical properties, such as the index of refraction or absorption coefficient, are dependent on the intensity of the incident light) have been used in passive optical devices designed to reduce or limit the fraction of light transmitted through the device as the incident intensity is increased.
The simplest optical geometry used in such a device is a dual lens (focus and recollimate) arrangement, chosen because of the large optical magnifications (high light intensities) which can be achieved in the nonlinear material, also because it can offer a wide field of view (critical for eye vision), and finally because it is a common scheme which occurs in many devices of interest to the military (such as, for example, periscopes, binoculars, gunsights and missile guidance systems) and to commerce (such as, for example, various laser applications).
The first lens (focusing lens) of such a dual lens arrangement focuses the incident beam onto a suitable material to maximize the nonlinear optical effects of the available energy. At low intensities, the nonlinear element has little effect on the beam and a second lens (imaging lens) of such a dual lens arrangement recollimates the light for transmission to the eye or sensor optics. However, it should be noted that in practice a second dual lens arrangement would be required to reinvert the image for vision applications. In combination with the lenses, entrance and collecting apertures establish the relevant f/number of the arrangement. When adjusted to the same size as the entrance aperture, the collecting aperture passes substantially 100% of the low intensity light. At high intensities, the nonlinear element defocuses the light, overfilling the collecting aperture, which spatially truncates and limits the magnitude of the transmitted beam.
The prior art in such defocusing limiters has utilized the electronic (.chi..sup.(3)) or orientational (Kerr) nonlinearities of semiconductors or organic compounds to defocus the incident light. There are a number of important requirements that must be met by the nonlinear material if it is to be used as the protective element in a defocusing optical limiter. These important requirements are:
1. It must possess a large, defocusing nonlinearity that, for the application of eye protection, is sufficient to limit the transmitted fluence to levels considered to be safe for retinal exposure (&lt;0.5 .mu.J/cm.sup.2). For sensor protection the transmitted fluence must be below the sensor damage threshold.
2. For eye protection, it must have a broadband spectral response to provide protection over all vision response wavelengths. For sensor protection, response over the entire ultraviolet/visible/infrared spectrum is required, depending on the sensor responsivity.
3. It must have high transmission of the low intensity light.
4. It must be compatible with low f/number optics.
5. It must have a large refractive index change (.DELTA.n.sub.sat &gt;0.1) before saturation occurs.
6. The material must have a fast yet persistent temporal response. In particular, limiting against Q-switched pulses in the range of 6 ns (nanoseconds) to 100 ns is generally regarded as the most important temporal regime and represents an absolute minimum material requirement.
7. It must possess either a high threshold for optical damage or the ability to recover between shots.
These seven requirements pose a severe test that has not been passed satisfactorily by currently available refractive materials. In particular, the low f/number requirement leading to the need for large .DELTA.n.sub.sat rules out most materials. The requirement for broadband response rules out the use of resonant semiconductor or organic nonlinearities. Nonresonant nonlinearities, although broadband, fail the requirements specified in paragraphs 1, 5 and 6 above.
Thermally induced refractive index changes in gases, solids and liquids are well understood. At high light intensities, refractive thermal blooming is accompanied by thermal aberrations which spatially redistribute the beam so that a significant amount of the energy originally in the center appears as rings at large angles with respect to the propagation direction of the light. These rings are conveniently blocked by an aperture thereby limiting the transmitted energy. A purely refractive thermal mechanism was proposed some time ago for the control of the output power of a cw (continuous wave) laser. However, it has been commonly assumed that the temporal response of a purely thermal mechanism is too slow to yield effective optical limiting of high energy, ns duration laser radiation. In fact, for a tightly focused beam the build-in time of the refractive nonlinear response can be on the order of a nanosecond.
The present inventors have described in the above cross-referenced, related U.S. application Ser. No. 08/251,146 now U.S. Pat. No. 5,491,579 a broadband thermal optical limiter capable of protecting eyes and sensors from sources of light of high intensity, such as Q-switched lasers. That limiter operated by the mechanisms of thermal defocusing (blooming) and thermal aberration. At low incident intensities light is transmitted by the limiter with little effect other than a small decrease in transmission (25% to 50%) due to a broadband absorbing dye. At higher light intensities thermal defocusing expands the central portion of the transmitted beam and redistributes most of the energy into rings at a large angle with respect to the propagation direction of the light. The rings are then conveniently blocked by an aperture (f/5 optics). The advantages of the broadband thermal limiter were: 1) The spectral response was truly broadband, extending from the blue to the near infrared (IR). 2) The transmitted fluence was limited to levels below the maximum permissible exposure level of the human eye. 3) The response time was sub-nsec for low f/# optics with protection for pulse durations up to tens of microseconds. 4) The damage threshold was very high.